Energy and angle resolved SIMS studies of CO on Ni(001)

Energy and angle resolved secondary ion mass spectra (SIMS) for CO chemisorbed on Ni(001) have been examined in detail. This system has been chosen as a model since it provides intense secondary ion yields and since the original surface geometry of the adsorbed CO is known by other techniques. Theoretical curves for the ejected atomic and molecular species have been generated using a classical dynamics procedure for computing the momentum dissipation of the 1000 eV incident Ar⁺ ion. We found that for Ni⁺ ion ejection the results agreed well with calculated spectra of the neutral particles where the CO is placed in a linear bonded site, if the neutral atom trajectories were modified by inclusion of an image force. The agreement was excellent for polar angle, azimuthal angle, and secondary particle kinetic energy distributions. Similar agreement was found for Ni ₂⁺ and NiCO⁺ species although the statistical reliability of the calculated curves was not as high as for the Ni⁺ species. The results provide convincing evidence that the classical dynamics model can provide a semiquantitative insight into the SIMS process. In addition, the presence of a relatively strong image force indicates that the ion must be formed very close to the surface. Finally, since agreement between theory and experiment was found over a wide range of conditions, the results suggest that the ionization probability of the ejecting particle is isotropic and only weakly dependent on particle velocity. These criteria impose a number of constraints on possible theories of ionization mechanisms.